Exhaust gas purifying catalyst compound, catalyst comprising said compound and method for preparing the compound

ABSTRACT

The invention is to provide (i) a catalyst which does not require an activation of catalyst components by means of a calcination which has become a hindrance in the way of obtaining a catalyst having a high activity through a conventional technology and in which catalyst the compositing of vanadium with molybdenum is contemplated more than enough; ii) a method for producing the catalyst; (iii) a catalyst having an activity, especially having an activity at low temperatures and a durability both greatly increased; (iv) a catalyst compound for purifying an exhaust gas, in which compound the ratio of vanadium atom to molybdenum atom (V/Mo) is 3/2 or close thereto and which compound is expressed by the rational formula (NH 4 ) x Mo 2 V x O (3x+6)  wherein x is 2.8 to 3.2; and (v) a method for producing the catalyst compound through a step for reacting molybdenum oxide (MoO 3 ) with ammonium metavanadate (NH 4 VO 3 ) in the co-presence of water for a prescribed period of time.

TECHNICAL FIELD

The present invention relates to a catalyst for purifying an exhaustgas. More specifically, the present invention relates to a catalystexcellent in the activity at low temperatures and resistance to SOx, andexhibiting a high activity in the reducing reaction of nitrogen oxides(NOx) with ammonia (NH₃) and oxidative decomposition reaction ofdioxins, and relates to a method for producing the catalyst.

BACKGROUND ART

NOx contained in exhaust gases or flue gases discharged from powerplants, various factories, automobiles, and others are causative agentsfor photochemical smog and acid rain. As an efficient method forremoving the NOx, an exhaust gas or flue gas denitration method by aselective catalytic reduction using ammonia (NH₃) as reducing agent haswidely been employed with thermal power plants being the places wherethe method is most frequently used. As the catalyst used for such anexhaust gas or flue gas denitration method, a titanium oxide (TiO₂) typecatalyst containing vanadium (V), molybdenum (Mo), or tungsten (W) as anactive component has been used. Especially, a catalyst containingvanadium as one active component has become a mainstream of currentdenitration catalysts since the catalyst is not only high in activitybut also small in deterioration due to the impurities contained in theexhaust gases and usable at temperatures including temperatures lowerthan those used heretofore (Laid-open Japanese Patent Publication No.Sho 50-128681 and others). The catalyst has been used after being moldedusually into a honeycomb-like or platelike shape, and various methodsfor producing the catalyst has been devised.

Besides, a fact that dioxins having a high toxicity are contained inexhaust gases discharged from incinerators burning municipal refuses orindustrial wastes has become a social problem in recent years. Thus, thecatalysts which perform a denitration reaction and oxidatively decomposedioxins at the same time have been invented.

Many of the catalysts described above are ordinarily prepared by (i) amethod in which particles of a titanium oxide, and particles of salts ofactive components such as V, Mo, and W of a catalyst or particles of anoxide are kneaded together with water, and the mixture thus obtained ismolded and calcined (kneading method), or (ii) another method in which amolded and calcined article of a titanium oxide is impregnated with asolution of a mixture of the salts of active components of a catalyst(impregnation method) (Laid-open Japanese Patent Publication No. Sho50-128681, Japanese Patent Publication No. Sho 53-34195, and others).

The kneading method and impregnation method described above both ofwhich fall within conventional technology can not always be said to bebest methods for preparing the catalysts when viewed from the aspect ofthe activity of catalysts, and many angles undesirable exist in themethods especially when the catalyst having a high activity at lowtemperatures are to be obtained.

The problems contained in the methods are enumerated with the problemsbeing separated into those belong to kneading method or those belong tothe impregnation method as follows:

1) Kneading Method

-   -   {circle around (1)} In order to obtain a catalyst (final        product) having a high activity, it is necessary to activate        added salts of active components contained in a molded catalyst        through calcination thereof. However, since a titanium oxide and        active components contained in the molded catalyst are sintered        by the calcination, it is difficult to obtain a catalyst (final        product) having a high activity at low temperatures.    -   {circle around (2)} So-called compositing effects of V with Mo        or W is not sufficient because particles of a titanium oxide,        and salts of active components such as V, Mo, and W, or        particles of an oxide once become a state in which all of them        coexist as they are by the kneading, and then they are        composited only after they were subjected to a calcination.        Accordingly, the kneading method leads to the formation of a        catalyst (final product) having a small improvement in the        durability and having a small resistance to SOx.        2) Impregnation Method    -   {circle around (1)} Like the kneading method, it is necessary to        activate the active components by calcining a molded catalyst.        Accordingly, it is impossible to avoid the active components        from being sintered, and thus a catalyst having a high activity,        especially a high activity at low temperatures can not be        obtained.    -   {circle around (2)} Since a catalyst molded and calcined in        advance is impregnated with active components in the        impregnation method, compositing of the active components become        easy compared with the kneading method. On the other hand,        however, the concentrations of active components become        different between the inside and the surface of a catalyst, it        is difficult to maintain the ratio of two or more active        components within a catalyst at a constant value, and thus most        suitable compositing effects can not be expected, because the        active components are adsorbed by a titanium oxide in the        process of the impregnation.

In order to overcome the problems in the background art described above,

-   -   (i) combined methods of a kneading method with an impregnation        method such as an impregnation method in which a titanium        oxide-molybdenum oxide carrier is impregnated with a vanadium        salt and another impregnation method in which a titanium oxide        carrier is impregnated sequentially with W, V, and others; and    -   (ii) improvements in the impregnation method have been        attempted.

However, it can hardly be said that sufficient-effects can be obtainedby those methods.

DISCLOSURE OF THE INVENTION

The subjects of the present invention are to provide a catalyst whichdoes not require the activation of catalyst components by a calcinationwhich has become a hindrance in the way of obtaining a catalyst having ahigh activity through conventional technology and in which catalyst thecompositing of vanadium with molybdenum to form a composite material iscontemplated more than enough, and a method for producing such catalyst,and to provide a catalyst having a high activity, especially having anactivity at low temperatures and a durability both greatly increased.

The method adopted in the present invention in order to achieve thesubjects described above is one in which (i) a molybdenum-vanadium(Mo—V) composite compound (hereinafter, “composite compound” issometimes referred to as “compound” for brevity) having a specificcomposition which is not necessary to be activated through a calcinationis used as an active component, and a carrier comprising a titaniumoxide as a main component is impregnated with the compound; or (ii) thecomposite compound is added to particles or powders of a titanium oxideand then kneaded.

Accordingly, the present invention is summarized as follows:

-   (1) A catalyst compound for purifying an exhaust gas, in which    compound the ratio of vanadium atom to molybdenum atom (V/Mo) is 3/2    or close thereto and which compound is expressed by the rational    formula    (NH₄)_(x)Mo₂V_(x)O_((3x+)6)    wherein x is 2.8 to 3.2.-   (2) A catalyst for purifying an exhaust gas, in which catalyst the    water soluble compound recited in paragraph (1) above is supported    on a carrier.-   (3) A catalyst for purifying an exhaust gas, which catalyst is    produced through a step for impregnating a titanium oxide carrier    with the water soluble compound recited in paragraph (1) above, or a    step for kneading powders of a titanium oxide together with the    water soluble compound.-   (4) The catalyst for purifying an exhaust gas according to    paragraph (2) or (3) above wherein the catalyst is produced by    further subjecting the catalyst recited in paragraph (2) or (3)    above to a calcination at a temperature lower than 500° C.-   (5) A process for producing the catalyst compound recited in    paragraph (1) above, which process comprises a step for reacting    molybdenum oxide (MoO₃) with ammonium metavanadate (NH₄VO₃) in the    co-presence of water for a prescribed period of time.-   (6) A composition used for a catalyst for purifying an exhaust gas,    which composition comprises the water soluble compound recited in    paragraph (1) above and a sol-like substance such as a silica sol.-   (7) A catalyst for purifying an exhaust gas, in which catalyst the    composition recited in paragraph (6) above is supported on a    carrier.-   (8) A process for producing a catalyst for purifying an exhaust gas,    which process comprises a step for having a mixture of the water    soluble compound recited in paragraph (1) above with a sol-like    substance such as a silica sol supported on a titanium oxide carrier    or a step for blending the mixture with powders of a titanium oxide,    after the water soluble compound was mixed with the sol-like    substance in advance.

The Mo—V composite compound used in the present invention is one whichwas found by the present inventors as a result of diligentinvestigations for solving the problems in the conventional technologydescribed above. The compound is a reddish brown substance produced byadding ammonium metavanadate (NH₄VO₃) and molybdenum trioxide (MoO₃)into water so that the ratio of vanadium atom to molybdenum atom (V/Mo)becomes 3/2 (or 6/4) or close thereto and then stirring them for aprescribed period of time (usually more than 10 hours). The compound ischaracterized by having a solubility as large as 170 g/l at a normaltemperature. Whereas the attempts to determine the structure of thecompound have been made by the present inventors, only the fact that thecompound is a reddish brown compound formed when ammonium metavanadatewhich is hardly soluble in water and molybdenum trioxide were added inwater and then stirred for a long period of time, has the ratio ofvanadium to molybdenum (V/Mo) of 3/2 or close to 3/2, and has a highsolubility is known at the present time. In this connection, attemptswere made by the present inventors to prepare a compound similar to thecompound of the present invention by using the combination of molybdenumtrioxide (MoO₃) with divanadium pentoxide (V₂O₅), ammonium metavanadate(NH₄VO₃) with ammonium molybdate ((NH₄)₆Mo₇O₂₄O₂₄.4H₂O), ammoniumparatungstate hexahydrate ((NH₄)₁₀H₁₀.W₁₂O₄₆.6H₂O) with ammoniummetavanadate, or ammonium metavanadate with tungsten trioxide (WO₃) asstarting raw materials. However, the formation of a compound having sucha high solubility as described above from the attempts using thecombinations was not noticed. Accordingly, the term “molybdenum-vanadium(Mo—V) compound” as used hereinafter is intended to mean a stable,reddish brown compound having a high solubility and formed when ammoniummetavanadate and molybdenum oxide were stirred together with water. Thecompound of the present invention has a high activity by itself. Thus,when the compound of the present invention was added to a titaniumoxide, it is not necessary to calcine the mixture anew to activate.Besides, since molybdenum and vanadium are formed into a stable compoundor composite in the present invention, the compound of the presentinvention is hardly erroded by the SOx contained in exhaust gases andprovide a catalyst having a high durability.

When the catalyst of the present invention is produced, an operation inwhich a carrier of a titanium oxide which carrier is prepared in advanceis impregnated with a solution containing the Mo—V compound describedabove at a prescribed concentration and then dried is adopted. Since theMo—V compound exhibits a high activity even immediately after thedrying, the activation of the compound through its calcination is notnecessary. However, a step of calcination may be added, when desired.

Further, it is possible-to-obtain the catalyst employing the Mo—Vcompound of the present invention can be obtained not only by theimpregnation method described above, but also by a method in which theMo—V compound is added to powders of a titanium oxide, molded, and thendried, and further subjected to a calcination, if necessary.

Moreover, since the Mo—V compound used in the catalyst of the presentinvention is very stable, the compound is not decomposed even when mixedwith a sol-like substance such as a silica sol. Thus, a catalyst highboth in activity and strength can be obtained by applying a solution ofthe mixture of the Mo—V compound and the sol-like substance to atitanium oxide carrier to have the mixture supported by the carrier.

As described above, the essence of the present invention resides in theuse of a novel composition of matter comprising Mo—V compound discoveredby the present inventors. Accordingly, the scope of the presentinvention is not limited by the fact that inorganic fibers, othercatalyst components, or inorganic or organic bonding agents are includedin the catalyst or titanium oxide. Besides, while a catalyst having ahighest activity and a longest durability can be obtained when theactivation by a calcination of the (molded) catalyst is not performed,the calcination may be conducted at a temperature lower than 500° C.,when required, depending on the strength and use conditions of thecatalyst. While the amount of the Mo—V compound to be permeated into atitanium oxide for impregnation of or to be mixed with the titaniumoxide is not limited, preferable results tend to be obtained usuallywhen the amount is selected so that the amount of the Mo—V compoundbecomes less than 20% (by weight, the same basis is also appliedhereinafter) and desirably less than 10% of the titanium oxide.

[Function]

In order to explain the functions in and the effects of the catalyst ofthe present invention, the problems of the catalysts obtained by aconventional kneading method or impregnation method are first described.FIG. 2(A) is a schematic diagram for illustrating the cross section of asurface layer of a catalyst obtained by a conventional kneading methodbefore it is subjected to a calcination. As will be understood from thediagram, V (vanadium) compound 1 and Mo (molybdenum) compound 2 added asactive components become particles of each compound at a step of dryingthem, and particles of the compounds and particles of a titanium oxideexist in a mixed state. In order that a catalyst at this state exhibitsan activity, the active components are necessary to be calcined tobecome oxides. Besides, in order that the particles of the V compoundand particles of the Mo compound existing at separated positions arecomposited to develop compositing effects such as an increase ofdurability of the catalyst, the components are necessary to be calcinedat a higher temperature to diffuse by heat thereby to react with eachother. When the catalyst was calcined, however, sintering of not onlythe active components but also the titanium oxide proceed. As theresult, a catalyst having a small specific surface is produced, and thusa high activity of the catalyst can not be expected. Further, thecompositing of the active components through the heat diffusion cannotbe said to be sufficient even in the aspect of the increase ofdurability of the catalyst since all the active components cannotuniformly react in the heat diffusion.

FIG. 2(B) is a schematic diagram for illustrating the cross section of asurface layer of a catalyst obtained by a conventional impregnationmethod in the step of the drying of the catalyst. Even in the case ofthe impregnation method, particles of each of the active components aredeposited or exist separately on the surface of particles of titaniumdioxide (TiO₂) 3 at the step of drying of the catalyst, and thus it isneedless to say that the activation or compositing of the activecomponents through a calcination is necessary. Besides, as a problempeculiar to the conventional impregnation method, a distribution of eachof the active components occurs within a catalyst due to the differencein the affinity (adsorptivity) between the titanium oxide and each ofthe active components contained in a solution of a mixture of thecomponents used for the impregnation. Accordingly, it becomes mostdifficult to maintain a certain ratio of the active components uniformlythrough a whole catalyst, and thus it is also difficult to obtain a highdurability through the compositing of active components with each other.

On the other hand, FIG. 1 is a schematic diagram for illustrating thecross section of a surface layer of a catalyst of the present invention.As will be clearly understood from FIG. 1, in a catalyst of the presentinvention, an active Mo component and an active V component exist on thesurface of titanium dioxide particles 3 as particles 4 in a compositedstate already at the stage where the drying of a catalyst was completed,because the catalyst of the present invention is produced byimpregnating a catalyst carrier with a solution of a Mo—V compound whichwas found anew by the present inventors and has a high stability, tomake the carrier support the Mo—V compound. Accordingly, it is notnecessary to composite the active compounds again through a calcination.Moreover, the catalyst of the present invention does not require theactivation through a calcination since the Mo—V compound supported onthe carrier has a high activity by itself. Accordingly, a clacinationstep causing the sintering of active components and a titanium oxide isunnecessary for the catalyst of the present invention. Keeping with suchan advantage, it becomes possible to obtain a catalyst having a highactivity, particularly an extremely high activity at low temperatureswhich requires a high surface area of a catalyst. Even when acalcination step is adopted in the process for producing the catalyst ofthe present invention for the reasons other than the activation orcompositing, the calcination can sufficiently be performed at a lowesttemperature necessary for its purpose, and it becomes possible to easilyobtain a catalyst having a high activity. Moreover, noteworthy is thefact that a specific ratio of Mo/V of the active components added ismaintained everywhere within the catalyst of the present invention.Based on this fact, not only a specific compositing treatment isunnecessary, but also it becomes possible to obtain a catalyst having ahigh activity and durability at all times according to the presentinvention.

More advantageous points of the present invention are that the Mo—Vcompound newly discovered is remarkably stable, that the compound is notbroken even when mixed with a silica sol or titanium dioxide (TiO₂), andthat the compound can be used as a solution of a mixture with a silicasol for the impregnation or can be used for the kneading with thetitanium dioxde. Based on these facts, it becomes possible to use theactive components together with a colloidal silica, which has a largeeffect of increasing strength, at the same time for the impregnation orkneading, and thus it becomes possible to considerably simplify themanufacturing processes of the catalyst.

Further, the Mo—V compound used in the present invention ischaracterized in that the compound is uniformly supported by a carriereven down to the inside of the catalyst since the affinity of thecompound for titanium dioxide is small. The catalyst of the presentinvention is advantageous even in this point since all the active pointsin a whole catalyst are employed in a denitration at a low temperaturearound 200° C. used for an exhaust gas such as an exhaust gas from amunicipal refuse incinerator. Besides, whereas the deterioration of adenitration catalyst with SOx tend to become remarkable at lowtemperatures, it becomes possible to obtain a catalyst having a highresistance to SOx in such an extent as that which was not expectableheretofore since the Mo—V compound supported is stable and does notreadily react with SOx.

In addition, the catalyst of the present invention exhibits highperformances even in the oxidative decomposition of chlorine containingorganic compounds, and is most suitable as a catalyst for denitrating anexhaust gas discharged from a municipal refuse incinerator and removingdioxins from the exhaust gas for which a catalyst is required to haveboth the resistance and the activity of oxidizing chlorine containingcompounds such as dioxins at the same time at around 200° CH.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for illustrating the cross section of asurface layer of a catalyst of the present invention and showing thestate of the distribution of active components in the catalyst.

FIG. 2 shows a schematic diagram similar to that of FIG. 1 showing thestate of the distribution of active components in a catalyst accordingto a conventional technology.

FIG. 3 is a graph comparing the performances of catalysts obtained inthe Examples of the present invention and Comparative Examples thereto,respectively.

In the drawings, the meanings of the referential numerals are asfollows:

-   -   1 . . . V compound, 2 . . . Mo compound, 3 . . . a titanium        oxide, and 4 . . . Mo—V compound used in the present invention.

BEST MODE FOR CARRYING THE INVENTION

Now, the present invention will be described in more detail withreference to specific examples. However, it should be understood thatthe scope of the present invention is by no means limited by suchspecific examples.

EXAMPLE 1

A slurry prepared by adding 40.6 g of molybdenum trioxide (MoO₃) and49.4 g of ammonium metavanadate (NH₄VO₃) into 410 g of water was gentlystirred at a room temperature for 20 hours to react both compounds witheach other thereby completely dissolve in the water. In the solutionthus obtained, the concentration of the Mo—V compound was about 18% byweight, and the ratio of the V atom to the Mo atom (V/Mo) in thecompound was 3/2. The reddish brown substance thus formed and dissolvedin the water can be expressed by the rational formula (NH₄)₃Mo₂V₃O₁₅.

COMPARATIVE EXAMPLE 1

A slurry prepared by adding 49.8 g of ammonium molybdate((NH₄)₆.Mo₇O₂₄.4H₂O) and 49.4 g of ammonium metavanadate (NH₄VO₃) into410 g of water was gently stirred at a room temperature for 20 hours. Inthis case, although the concentration of the solid substance in theslurry at the stage when the stirring was completed was about 18% byweight and the ratio of the V atom to the Mo atom (V/Mo) in the slurrywas 3/2 in common with the results in Example 1, the ammoniummetavanadate was not dissolved in the water and a reddish brown productwas not obtained.

COMPARATIVE EXAMPLE 2

A slurry prepared by adding 49.8 g of ammonium molybdate((NH₄)₆.Mo₇O₂₄.4H₂O) and 38.4 g of vanadium pentoxide (V₂O₅) into 410 gof water was gently stirred at a room temperature for 20 hours. In thiscase, although the concentration of the solid substance in the slurry atthe stage when the stirring was completed was about 18% by weight andthe ratio of the V atom to the Mo atom (V/Mo) in the slurry was 3/2 incommon with the results in Example 1, the vanadium pentoxide was notdissolved in the water and a reddish brown product was not obtained.

COMPARATIVE EXAMPLE 3

A slurry prepared by adding 65.4 g of tungsten trioxide (WO₃) and 49.4 gof ammonium metavanadate (NH₄VO₃) into 410 g of water was gently stirredat a room temperature for 20 hours in the attempt to react bothcompounds with each other thereby to completely dissolve in the water.Although the ratio of the V atom to the W atom (V/W) in the slurry was3/2 in common with the results in Example 1, both added compounds werescarcely dissolved in the water, and a reddish brown product was notobtained.

As will be clear from the results in Example 1, and Comparative Examples1 to 3, a reddish brown-substance formed in Example 1, and having theratio of the V atom to the Mo atom (V/Mo) of 3/2 and a high solubilityin water can be considered to be a specific substance formed by thereaction of ammonium metavanadate with molybdenum trioxide in water.Thus, it has been found that the specific compound is not formed when astarting raw material is different and that even when a tungsten oxidewhich is a homologue of the molybdenum compound is used in place of thelatter compound, the specific compound is not resulted.

EXAMPLE 2

A net-like product prepared by weaving twisted yarns each comprising1400 E glass fibers having a fiber diameter of 9 μm at a roughness of 10yarns/inch into a plain weaving was impregnated with a slurry containing40% of a titania, 20% of a silica sol, and 1% of a polyvinyl alcohol,and then dried at 150° C. to obtain a catalyst substrate imparted with astiffness.

On the other hand, 1.5 kg of a titanium oxide having a specific surfacearea of about 230 m²/g was added to 75 g of oxalic acid, water was addedthereto to form a paste-like substance, and then 300 g of silica.aluminafibers were added thereto and kneaded until a homogenous paste wasobtained in a separate step.

The paste thus obtained was placed between two sheets of the catalystsubstrates obtained by the procedures described above, and they werepassed through pressure rolls to obtain a sheet having a thickness of0.7 mm and containing the titania. This sheet was dried at a roomtemperature and then calcined at 400° C. for 2 hours to obtain a titaniacarrier.

This titania carrier was immersed in the solution of the Mo—V compoundsynthesized in Example 1 to support the Mo—V compound and then dried at80° C. for 2 hours. The contents of molybdenum trioxide (MoO₃) anddivanadium pentoxide (V₂O₅) in the catalyst thus obtained were 4.8% byweight and 4.5% by weight based on the amount of titanium dioxide(TiO₂), respectively.

COMPARATIVE EXAMPLE 4

A net-like product prepared by weaving twisted yarns each comprising1400 E glass fibers having a fiber diameter of 9 μm at a roughness of 10yarns/inch into a plain weaving was impregnated with a slurry containing40% of a titania, 20% of a silica sol, and 1% of a polyvinyl alcohol,and then dried at 150° C. to obtain a catalyst substrate imparted with astiffness.

On the other hand, 88.3 g of ammonium molybdate and 75 g of oxalic acidwere added to a mixture of 1.5 kg of a titanium oxide having a specificsurface area of about 230 m²/g and 86.7 g of ammonium metavanadate,water was added thereto to form a paste-like substance, and then 300 gof silica.alumina fibers were added thereto and kneaded until ahomogenous paste was obtained in a separate step.

The paste thus obtained was placed between two sheets of the catalystsubstrates obtained by the procedures described above, and they werepassed through pressure rolls to obtain a sheet having a thickness of0.7 mm. This sheet was dried at a room temperature and then dried at 80°C. for 2 hours to obtain a catalyst.

The contents of molybdenum trioxide (MoO₃) and divanadium pentoxide(V₂O₅) in the catalyst thus obtained were 4.8% by weight and 4.5% byweight based on the amount of titanium dioxide (TiO₂), respectively.

EXAMPLES 3 to 6

The same catalysts as that obtained in Example 2 were calcined at 200,300, 400, and 500° C., respectively, for 2 ours to prepare separatecatalysts.

COMPARATIVE EXAMPLES 5 to 8

The same catalysts as that obtained in Example 4 were calcined at 200,300, 400, and 500° C., respectively, in the same manner as inComparative Examples 2 to 5 to prepare separated catalysts.

COMPARATIVE EXAMPLES 9 and 10

Ammonium metavanadate in an amount of 8.67 g and 8.83 g of ammoniummolybdate were dissolved in 41 cm³ of 10% aqueous solution of monomethylamine. In the solution thus obtained was immersed the same titaniacarrier as that used in Example 2 to impregnate, and then the carrierwas dried first at a room temperature and then at 80° C. to obtain thecatalyst of Comparative Example 9.

The same catalyst as that of Comparative Example 9 was further calcinedat 500° C. for 2 hours to obtain the catalyst of Comparative Example 10.

Each of the catalysts obtained by Examples 2 to 6 and ComparativeExamples 4 to 10, respectively, were cut into strips of 20 mm×100 mm,and then subjected to denitration tests under the conditions as shown inTable 1. The results thus obtained are shown in FIG. 3 as the relationbetween the calcining temperatures of catalysts and denitration ratios.In this connection, the drying temperature was assumed to be a portionof the calcination temperature, and the data of the catalysts subjectedonly to drying are included in FIG. 3.

As will be clear from FIG. 3, the catalysts of the present inventionexhibit an extremely high activity. In addition, whereas the catalystsof Comparative Examples do not exhibit a high activity unless they werecalcined at a temperature higher than 400° C., the catalysts of thepresent invention, for example, even the catalyst obtained by Example 2in which the calcination was not conducted at all exhibited the sameextent of excellent performances as those exhibited by calcinedcatalysts. From this fact, it is evident that the catalysts of thepresent invention are not necessary to be subjected to such anactivating treatment as a calcination, and thus that the catalysts ofthe present invention have large advantages in the activity andproduction cost of catalysts.

TABLE 1 Temperature (° C.) 200 Aerial velocity (m/h) 17 Gas compositionNO 200 NH₃ 240 O₂ 10 CO₂ 6 H₂O 6

On the other hand, the catalysts obtained by Examples 2 and 6, andComparative Examples 4, 8, 9, and 10, respectively, were exposed to anexhaust gas containing 200 ppm of SO₂ at 200° C. for 100 hours todetermine the extents in which the activities of the catalysts arelowered by the SO₂. That is, the performances of the catalysts at 200°C. before and after the exposure to the SO₂ containing gas weredetermined under the conditions shown in Table 1. The results thusobtained are shown in Table 2.

TABLE 2 Denitration ratio after Initial denitration subject to SO₂Catalyst ratio (%) treatment (%) Example 2 92 85 Example 6 89 80Comparative 27 5 Example 4 Comparative 71 20 Example 8 Comparative 21 2Example 9 Comparative 56 14 Example 10

As will be understood from Table 2, any of the catalysts of ComparativeExamples were considerably lowered in activity by the exposure to theSO₂ containing gas. On the contrary, the catalysts of the Examples wereextremely small in the lowering of the activity. Particularly, it can befound that the catalyst of Example 2 is smallest in the lowering of theactivity compared with other catalysts. This is considered to be due tothe fact that the catalysts of Examples are not easily affected by SOxin addition to the fact that the compounds are supported in the catalystin the form of Mo—V compound and thus the compositing of molybdenum withvanadium is sufficient.

As described above, it was found that not only the catalysts having ahigh activity can be obtained without a calcination according to thepresent invention, but also the catalysts of the present are remarkablyexcellent even in the aspect of durability.

In order to describe the effects of the present invention morespecifically, other examples are shown below.

EXAMPLE 7

A catalyst was prepared by the same manner as in Comparative Example 4with the exception that the solution of the Mo—V compound prepared inExample 1 was used instead of the solution of ammonium molybdate andammonium metavanadate in Comparative Example 4.

EXAMPLE 8

To the same solution of the Mo—V compound as that prepared in Example 1was added 500 g of a colloidal silica (produced by Nissan ChemicalIndustry Co., Ltd.; trade name: Silica sol-o) containing SiO₂ in anamount of 20% by weight to obtain a solution of a mixture of bothcomponents. By using this solution, a catalyst was prepared by the samemanner as in Example 2.

COMPARATIVE EXAMPLE 11

A solution was prepared by adding 50 g of a colloidal silica (Silicasol-o described above) to the same aqueous solution of monomethyl aminecontaining a mixture of ammonium metavanadate and ammonium molybdate asused in Comparative Example 9. By using the solution thus obtained, acatalyst was prepared in the same manner as that in Comparative Example9.

EXAMPLE 9

Denitration activity was determined at 200° C. by using the samecatalyst as that obtained by Example 4, and the oxidation ratio ofchlorobenzene by the catalyst was determined at the same time by addingchlorobenzene as a quasi dioxin substance in an amount of 10 ppm to theexhaust gas.

Performance tests of the catalyst obtained by Example 7 were conductedin the same manner as that used in Example 2. The results thus obtainedare shown in Table 3 together with the results in Example 2. It can beunderstood from Table 3 that even when a Mo—V compound was used as astarting raw material for the catalyst in the kneading method as shownin the line for Example 7 in Table 3, the same extent of high catalystperformances are obtained as in the case of the impregnation method.

TABLE 3 Method for adding Mo-V Denitration ratio Catalyst compound (%)Example 2 Impregnation method 92 Example 7 Kneading method 91

Besides, the activities at 200° C. and the results of bending strengthtests of both catalysts obtained by Example 8 and Comparative Example11, respectively, are collectively shown in Table 4. As will beunderstood from Table 4, the catalyst of Comparative Example 11 is notonly low in the activity but also has an extremely low value in thebending strength compared with the catalyst of Example 8. This ispresumed to be due to the fact that the silica sol and the vanadium saltwere changed to a gel-like substance by the mixing of the aqueoussolution of a mixture of the molybdenum salt and vanadium salt with asilica sol, and thus the gel-like substance was not penetrated into atitania carrier. As will be clear from this example, according to thepresent invention, it is possible to make a carrier support a strengthincreasing agent of a catalyst and an active component thereon at thesame time since the Mo—V compound of the present invention canarbitrarily be mixed with a silica sol, and thus, catalysts having ahigh strength and a high activity can be obtained through simpleprocedures.

TABLE 4 Denitration ratio Bending strength Catalyst (%) (kg/cm²) Example8 72 140 Comparative 14 60 Example 11

As a result of the tests conducted in Example 9, it was also found thatcatalysts of the present invention exhibit such a high activity foroxidatively decompose chlorobenzene as 80% or more. As described above,the catalysts of the present invention are not only excellent in theactivity at such a low temperature as 200° C., but also are hardlydeteriorated by the SOx contained in exhaust gases from refuseincinerators, and are excellent in the activity for oxidizingchlorine-containing organic compounds which are quasi dioxin substances,in addition. While it is desired to decompose dioxins and to performdenitration at the same time in refuse incinerators number of which isincreasing more and more in recent years, the catalysts of the presentinvention are remarkably excellent ones matched with such social needs.

INDUSTRIAL APPLICABILITY

According to the present invention, a catalyst compound which has acatalytic activity but does not need an activating step by a calcinationcan be obtained. A catalyst having the compound supported on a carrier,or included therein by kneading is excellent in the activity at lowtemperatures, durability, and the activity for decomposing dioxins, andthus it becomes possible to make refuse incinerators, apparatuses forpurifying exhaust gases, and the likes highly efficient. Besides, sincea calcination as a treatment for activating the active components of acatalyst become unnecessary, not only simplification of manufacturingsteps of a catalyst becomes possible, but also it becomes possible todesign a catalyst with the emphasis being placed on the catalystactivity, and various improvements in the performances and economicalaspects of the catalyst become possible. Further, the Mo—V compound usedin the present invention is not only stable even when mixed with asol-like substance such as a silica sol, but also does not form agelation of a sol. Accordingly, it is possible to support an activecomponent at the same time in an impregnation step of a catalyst with asilica sol conducted for the purpose of increasing the strength of thecatalyst, and it becomes possible to obtain a catalyst having a highstrength and a high catalytic activity through simple steps.

1. A catalyst compound for purifying an exhaust gas, in which compoundthe ratio of vanadium atom to molybdenum atom (V/Mo) is 1.4 to 1.6 andwhich compound is expressed by the rational formula(NH₄)_(x)MoV_(x)O_((3x+6)) wherein x is 2.8 to 3.2.
 2. A process forproducing the catalyst compound defined in claim 1, which processcomprises a step for reacting molybdenum oxide (MoO₃) with ammoniummetavanadate (NH₄VO₃) in the presence of water for a prescribed periodof time.
 3. A catalyst for purifying an exhaust gas, comprising a watersoluble compound supported on a carrier, the water soluble compoundbeing expressed by the rational formula(NH₄)_(x)Mo₂V_(x)O_((3x+6)) wherein x is 2.8 to 3.2 and the ratio ofvanadium atom to molybdenum atom is 1.4 to 1.6.
 4. The catalyst forpurifying an exhaust gas according to claim 3 wherein the catalyst isproduced by further subjecting the catalyst to a calcination at atemperature lower that 500° C.
 5. A composition used for a catalyst forpurifying an exhaust gas, which composition comprises a water solublecompound and a sol-like substance, the water soluble compound beingexpressed by the rational formula(NH₄)_(x)Mo₂V_(x)O_((3x+6)) wherein x is 2.8 to 3.2 and the ratio ofvanadium atom to molybdenum atom is 1.4 to 1.6.
 6. A catalyst forpurifying an exhaust gas, in which catalyst the composition defined inclaim 5 is supported on a carrier.
 7. A composition according to claim5, wherein the sol-like substance is a silica-sol.
 8. A process forproducing a catalyst for purifying an exhaust gas, which processcomprises a step for having a mixture of a water soluble compound with asol-like substance supported on a titanium oxide carrier, or a step forblending the mixture with powders of a titanium oxide, after the watersoluble compound was mixed with the sol-like substance in advance, thewater soluble compound being expressed by the rational formula(NH₄)_(x)Mo₂V_(x)O_((3x+)6) wherein x is 2.8 to 3.2 and the ratio ofvanadium atom to molybdenum atom is 1.4 to 1.6.
 9. A process accordingto claim 8, wherein the sol-like substance is a silica-sol.
 10. Acatalyst for purifying an exhaust gas, which catalyst is producedthrough a step for impregnating a titanium oxide carrier with a watersoluble compound, or a step for kneading powders of a titanium oxidetogether with the water soluble compound, the water soluble compoundbeing expressed by the rational formula(NH₄)_(x)MO₂V_(x)O_((3x+6)) wherein x is 2.8 to 3.2 and the ratio ofvanadium atom to molybdenum atom is 1.4 to 1.6.
 11. The catalyst forpurifying an exhaust gas according to claim 10 wherein the catalyst isproduced by further subjecting the catalyst to a calcination at atemperature lower than 500° C.